Rescue Weed Control in Direct-Seeded Rice

Pendimethalin works as a soil layer, not as a spray on leaves. It must remain in the top 2–3 cm as a continuous treated zone through which weed seedlings emerge. Three things went wrong here:

• Rain of 42 mm fell within hours. On a sandy loam with low organic carbon, adsorption is weak, so the herbicide moved with the wetting front below the weed seed germination zone, and some was lost with runoff.
• No irrigation or light incorporation followed, so the layer was never stabilised at the intended depth.
• There was no crop canopy or standing water to suppress the second flush.

The species mix confirms it. Echinochloa germinates in repeated flushes from the surface, Leptochloa chinensis is a late, tolerant germinator, and Cyperus rotundus emerges from tubers 5–10 cm deep that a surface herbicide layer never reaches. Ludwigia and Eclipta come up in the wetter patches after the rain.

Timing makes it urgent. The critical weed-free period in direct-seeded rice runs roughly from 20 to 45 days after sowing. At 25 days the crop is in the middle of that window, and every day of competition now removes tillers that cannot be replaced later. Yield loss can reach 40–60% in an uncontrolled DSR field, which is why a corrective treatment is justified rather than a wait-and-watch approach.

Do not tank-mix the two products. Bispyribac-sodium is a grass-and-sedge herbicide whose uptake is damaged by mixing with 2,4-D; 2,4-D sodium salt at this stage causes onion-leaf, twisted stems, delayed flowering and sterility in rice, and it is weak on Echinochloa. It does not "finish everything" - it adds crop injury and antagonism. Reject it.

A workable programme:

• Bispyribac-sodium 10% SC at 25 g a.i./ha, i.e. 250 ml/ha, in 200–250 L water with a non-ionic surfactant, applied now at 3–4 leaf. It must go on before the crop passes 6 leaf; later application injures the crop and misses the weeds.
• Add chlorimuron-ethyl + metsulfuron-methyl 10% + 10% WP at 4 g a.i./ha (40 g/ha) for Ludwigia and Eclipta, provided it is a rice-only product and the field is not near a sensitive crop.
• Leptochloa needs a separate grass treatment: fenoxaprop-p-ethyl 9.3% EC at 60 g a.i./ha or cyhalofop-butyl 10% EC at 80–100 g a.i./ha, applied 7–10 days after the bispyribac spray - never in the same tank.
• For Cyperus in the patches, halosulfuron-methyl 75% WG at 67.5 g a.i./ha, or hand pulling before tuber chains form.

One caution. Herbicide cost per hectare is comparable to two days of hired labour, so use the remaining labour on the patches rather than spraying the whole field twice. Apply in the morning with clean water and no drift to neighbouring vegetables.

Fix the establishment system, not only the herbicide. The failure began with a loose, unpuddled, unrolled seedbed and a herbicide layer that had nothing to hold on to.

Preventive package for the next kharif:

• Prepare a stale seedbed: irrigate, let weeds germinate, then spray paraquat or glyphosate or do a shallow tillage 7–10 days before sowing, and sow into the clean bed.
• Increase seed rate to 80 kg/ha and reduce row spacing to 15 cm so the crop canopy closes over the rows by 30 days; a dense canopy is the cheapest weed control in DSR.
• Use a pre-emergence option suited to the soil: pretilachlor 50% EC at 0.75 kg a.i./ha with safener applied to moist soil 3–5 days after sowing, or pendimethalin followed immediately by a light irrigation so the layer is set before rain.
• Practise brown manuring: sow dhaincha with the rice and kill it with 2,4-D at 25–30 days; the dead mulch suppresses later flushes.
• Level and roll the field so moisture is uniform and no patch escapes treatment.

Economic reasoning. A ₹1,500–2,000 preventive programme plus denser crop is cheaper than a rescue spray plus the yield already lost at 25 days. Keep a written field log of herbicide, dose, water volume and rain timing; the same mistake repeated twice is a management failure, not bad luck.